The respiration or respiratory rate (RR) or breathing frequency, is the rate (frequency) of ventilation, that is, the number of breaths (inhalation-exhalation cycles) taken within a set amount of time (typically 60 seconds).
Respiration rates may increase with fever, illness, or other medical conditions. Consequently, respiration rate (RR) is recognized as an important clinical parameter. Respiration rate is typically measured in a number of different ways depending upon the clinical setting. The measurement may be performed continuously, for example using: end-tidal CO2 (ETCO2) monitors, EKG-based trans-thoracic impedance systems, nasal thermistors, abdominal and chest bands. Continuous measurements involve specialized and/or obtrusive equipment and hence RESPIRATION RATE is normally available from these devices only when they have been specified for another clinical purpose. In contrast, manual counting typically used as an intermittent spot check made during patient observation and is not a continuous measurement.
Photoplethysmography or photoplethysmographic (PPG) systems have been used in an attempt to measure various blood flow characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and the rate of blood pulsations corresponding to each heartbeat of a patient. PPG systems use non-invasive, electro-optical methods that provide information about the volume of blood flowing in a test region close to the skin of body. For example, a PPG signal may be obtained by illuminating the region of interest of body and reflected or transmitted light. In generating the PPG signal, the wavelength A of a light source on one side is placed in a protrusion, for example a finger and a photo-detector or PPG sensor may be placed to other side of the source to capture the transmitted light.
Typically PPG measurement systems include an optical sensor for an attachment to the tip of patient's appendage (e.g., a finger, earlobe and others). The sensor directs light signals into the appendage where the sensor is attached. Some portion of light is absorbed and a remaining portion passes through patient tissue. The intensity of the light passing through the tissue is monitored by the optical sensor. The intensity related signals produced by the sensor are used to compute blood parameters.
PPG sensor can be used in reflection mode (reflecting off of tissue) or in transmission mode (transmitting through the tissue, such as an ear lobe). Normally, a wavelength A in the near infrared space is used because that wavelength generates the strongest modulation of the signal due to light absorption in the haemoglobin in the blood.
It is commonly believed that respiratory activity may cause the PPG to modulate in three fundamental ways. These are:                (1) Baseline (DC) modulation: Changes in venous return secondary to changes in intrathoracic pressure throughout the respiratory cycle cause a baseline DC modulation of the PPG signal. During inspiration, decreases in intrathoracic pressure result in a small decrease in central venous pressure increasing venous return. The opposite occurs during expiration. As more blood is shunted from the low pressure venous system at the probe site and the venous bed cyclically fills and drains, the baseline is modulated accordingly.        (2) Pulse amplitude modulation: Decreased left ventricular stroke volume, due to changes in intrathoracic pressure during inspiration, leads to decreased pulse amplitude during this phase of respiration.        (3) Respiratory sinus arrhythmia (RSA): This is a variation in heart rate that occurs throughout the respiratory cycle. Specifically, it has been well-documented that heart rate increases during inspiration and decreases during expiration. The presence of RSA is influenced by several factors including age, disease status, and physical fitness. While the precise mechanisms of RSA remain controversial, in general, it seems to be the result of autonomic nervous system activity fluctuation during respiration.        
The respiratory components often appear concurrently with a range of other low frequency artifacts due, for example, to patient movement, vasomotion or blood pressure changes.
Exact extraction of respiration information from PPG signals currently requires advanced and complex signal processing capabilities coupled with a full analysis of the character of respiratory modulations within the PPG. Such processing capability is beyond the computational processing power of small wearable and unobtrusive technology.
What is needed, therefore, are devices and methods that can accurately, but relatively simply estimate respiration rate from PPG signals so such devices can continuously monitor respiration rate, but in a non-intrusive and convenient manner. Specifically, methods and devices that use smaller processors implemented in wearable technology need simpler computational methods for estimating respiration rate.